The invention relates to a grip for the performance of tensile tests on elongated specimens, wherein in at least the face of the grip which engaged with the elongated specimen roughnesses are applied. The invention also relates to a grip holder and a tensile testing apparatus incorporating such a grip.
By roughnesses is meant areas which protrude with respect to the plane of the grip.
The elongated specimens are characterized by a dimension in the longitudinal direction which is many times greater than the dimensions of the cross-section. Examples of such elongated specimens are wires, cables and cords
Such elongated specimens are very frequently subjected :o tensile tests to determine the tensile strength, breaking strength and other parameters such as elongation, constriction, etc. To this end a certain length of an elongated specimen is gripped in the grip at both ends and subjected to an increasing tensile force until the elongated specimen breaks.
Despite repeated efforts in the past, the grips with which the elongated specimen are gripped continue to create problems.
A first problem occurs particularly during the testing of hard wires such as oil-hardened steel wires. In these tests, the grip is subject to severe wear and requires replacement after only ten tests have been performed This leads to a higher cost due to the high consumption of grips In addition, time is lost in the replacement of grips.
A second problem is the problem of breakages in the grip. The specimen to be tested, for example a steel wire, breaks inside the grip or just outside it. The tensile strength at which this occurs is not necessarily indicative of the actual tensile strength of the specimen, with the result that the tensile test is not reliable and has to be repeated.
In the present state of the art attempts have already been made to substitute the material from which the grips are made with a harder material which would be more resistant to wear. This has not, however, always led to more favourable results since the roughnesses on the grip, due to the extra hardness of the material, are also more brittle, break off more easily and lead to more rapid wear.